On period distribution of RR Lyr type variables in the globular cluster M3

Abstract

Evolutionary calculations of population II stars with chemical composition of the globular cluster M3 were carried out under various assumptions about the initial stellar mass ($0.809M_\odot\le M_\mathrm{ZAMS} \le 0.83M_\odot$) and the mass loss rate parameter in the Reimers formula ($0.45\leη_\mathrm{R}\le 0.55$). In general, 30 evolutionary tracks of the horizontal branch stars were computed. Selected models of evolutionary sequences were used as initial conditions for solution of the equations of hydrodynamics that describe radial stellar oscillations. Hydrodynamic models of RR Lyr type stars were computed for the core helium burning stage as well as for the preceding pre--ZAHB stage. Analytic relations for the effective temperature of the instability strip edges as a function of stellar luminosity are obtained. Theoretical histograms of the period distribution of RR Lyr type variables were produced for each evolutionary sequence using Monte--Carlo simulations based on the consistent stellar evolution and nonlinear stellar pulsation calculations. A satisfactory agreement with observations (i.e. the greater number of RRab variables) was found for the evolutionary sequence $M_\mathrm{ZAMS} = 0.811M_\odot$, $η_\mathrm{R}=0.55$ with the number fraction of fundamental mode pulsators $\approx 75\%$. At the same time the mean period of fundamental mode pulsators ($\langleΠ\rangle_0=0.79$ day) is substantially greater compared to the observational estimate of $\langleΠ\rangle_\mathrm{ab}$.